Create new PerlySense::Class object given the class found at $row,
$col in $file.

If there was no package started yet at $row,
$col,
but there is one later in the file,
use the first one instead (this is when you're at the top of the file and the package statement didn't happen yet).

Return new object,
or undef if no class was found,
or die if the file doesn't exist.